The present invention relates to measuring and testing methods and apparatus and more particularly to programmable, automatic circuit testers and in-circuit component testers.
The testing of circuits and components may be classified into the general categories of analog and digital testing although distinctions between the categories is at times imprecise.
The category of analog testing refers to excitation with and measurement of signals that are quantitative and continuous in nature. For example, measurements of voltage, current, frequency and time are typically classified as analog measurements. Analog testing is sometimes referred to as dynamic testing when time or some other dynamically variable parameter is involved or is sometimes referred to as static testing when a fixed, non-dynamincally variable parameter is involved.
The category of digital testing refers to the examination of logic patterns to determine whether they are binary 1's or 0's without regard to their quantitative values as long as they fall within gross threshold ranges. Digital testing is sometimes referred to as pattern testing.
Components are building blocks for forming circuits. For example, resistors, capacitors, inductors, delay lines, amplifiers, logical gates, and integrated circuits on semiconductor chips are all typical components. Components are characterized by having a common support or housing with one or more terminals for use in connecting the components into a larger circuit. Components are commonly manufactured to contain several like elements. For example, four NAND gates are typically manufactured in a single package as a component. Each logical element; that is, each NAND gate, is separately connectable in a circuit.
Circuits are a plurality of interconnected components which operate to perform functions. The points at which components are connected in a circuit are called nodes. Circuits perform functions by receiving input signals one or more selected input nodes to produce output signals on one or more selected output nodes. The input and output nodes are called external nodes. The input and output nodes may or may not be the same nodes. Either input or output nodes may not be present in some components.
In addition to the input and output (I/O) nodes, circuits typically include one or more internal nodes which are the connection points for components which are not utilized to conduct input or output signals to or from the circuits. Components such as semiconductor chips typically include component circuits fabricated on a single wafer from a plurality of non-discrete internal components such as diodes and transistors. Many of the internal nodes within integrated circuit chips typically are not utilized for connection of input or output signals. A limited number of I/O nodes (external nodes) are available from a semiconductor chip for connection of the chip in a larger circuit where the larger circuit typically includes other chips and components. Of course, if desirable, chips, may be constructed with one or more internal nodes having connections which, like I/O nodes, are available for external connection. The I/O nodes of conventional circuits are identified as "pins" and have "pin numbers".
The relationship between non-discrete components and a semiconductor chip is the same relationship as between discrete components and a circuit board on which the discrete components are mounted and connected. The I/O nodes (internal nodes) on the semiconductor chip and the I/O nodes on a circuit board must be available for connection of signals. The internal nodes of semiconductor chips and of circuit boards are generally not available for making connections. Although circuit boards are more readily adapted, using test fixtures, for connections to internal nodes, semiconductor chips can be designed to make internal nodes available for connection.
The process of functional testing consists of applying at the input terminals of a device under test, (either a circuit or a component) the same signals normally applied in its intended application and determining whether or not the signals on the output terminals are acceptable for normal operation. In digital functional testing, a digital output is examined to determine if it matches the expected pattern of 1's and 0's. In analog functional testing, the output signals are measured so it can be determined if their incremental levels fall within acceptable limits of time, voltage, current or other parameters.
The process of component in-circuit testing consists of testing a component while it is connected in its normal manner within a circuit. The in-circuit testing of a component requires access to the nodes by which the component is interconnected in the circuit. The nodes may be internal or external or both.
Whenever a component is in-circuit tested, special problems arise because of the interaction of the component with the remainder of the circuit to which it is interconnected. Generally, some form of isolation or guarding is required in order to satisfactorily in-circuit test components. Additionally, a judicious selection of the test signals must be made in order to insure that either the component under test or the circuit it is interconnected with is not damaged.
In order to insure that the test signals do not damage the components of the circuit under test, the time duration of test signals has been made short, several microseconds or less, in order to limit the total delivered power.
Because of the high number of components and circuits which are manufactured, test equipment to be fully useful and economical must be automated, high-speed, highly reliable, and flexible. Automated testers have been available for the functional testing of circuits and components.
The above cross-referenced application provides a method and apparatus for automatic, programmed, in-circuit component testing and functional testing. In that invention, the programs were correlated with commanded measurement parameters and measurement sequences.
There is a need, however, for improved test apparatus and methods for the automatic, programmed, in-circuit testing of individual logic elements in a circuit under test.